Field of the Invention
Embodiments of the invention relate generally to the field of optical communications. More particularly, an embodiment of the invention relates to compensation of distortion from SBS/IIN suppression modulation.
Discussion of the Related Art
Directly modulated lasers are commonly used in hybrid fiber-coax systems to transmit RF signals over long distances using optical fibers. These lasers are usually single mode and operate at wavelengths near 1310 nm or 1550 nm. The advantage of 1310 nm is most installed fiber has zero dispersion around this wavelength. Zero dispersion is important because the wavelength of directly modulated lasers fluctuates or chirps with modulation. This can lead to non-linear transmission effects if fiber dispersion is not zero.
The advantage of 1550 nm is most installed fiber has minimum loss around this wavelength. So 1550 nm can be used to transmit signals over longer distances than 1310 nm. In addition, the non-zero dispersion that most installed fiber exhibits around 1550 nm helps mitigate four wave mixing effect, allowing 1550 nm to be more easily used for wavelength division multiplexing. However, the non-zero dispersion interacts with laser chirp to produce intermodulation distortion of the transmitted signal. One way to mitigate the effects of chirp-dispersion induced distortion is to reduce the chirp of the optical source. Directly modulated optical sources with low chirp include, but are not limited to, special low chirp DFB lasers as well as external cavity lasers.
Reducing chirp too much or under-modulating the laser can lead to an increase in noise and distortion from SBS and/or IIN effects. To help compensate for this, a low frequency SBS/IIN suppression modulation can be added to the laser. This can be in the form of a bias modulation and/or a thermal modulation. A thermal modulation is preferred because it will not decrease clipping margin. However, obtaining a fast thermal modulation can be difficult. Therefore, a low frequency bias modulation may be more practical. Fortunately, a low frequency bias modulation will produce a similar effect as thermal modulation. The chirp from low frequency modulation is typically dominated by thermal effects caused by junction heating and will provide a much larger increase in laser linewidth than can be achieved for the same modulation amplitude at RF frequencies. So a significant increase in linewidth can be obtained with only a small decrease in clipping margin.
Various predistortion schemes have been proposed to compensate for intermodulation distortion caused by chirp-dispersion interaction of the transmitted RF signal. These schemes all involve compensation of only intermodulation products of the RF signal to be transmitter i.e. distortion products of the RF signal mixing with itself. The purpose of this invention is to provide means compensate for intermodulation products of the RF signal mixed with a low frequency SBS/IIN suppression signal whether it be from thermal or bias modulation of the optical source.